We can't produce a true vacuum, so how do we know how fast light travels through 1?

One reason is this: at low pressures, the electromagnetic properties (electric permittivity ε0 and magnetic permeability μ0) of air (for example) vary very little with pressure, so making the extrapolation to complete void gives the same result. You know that light's speed is related to these properties: c = 1/Sqrt(μ0ε0)